1. Field of the Invention
The present invention relates to a current and voltage detection circuit that detects a current and a voltage, which are supplied from a power supply, and to a current control circuit that performs current control corresponding to the current and the voltage, which are detected by the current and voltage detection circuit. For example, the present invention relates to a technology effective for use in a charge control device that charges a secondary battery.
2. Background Art
For a charge device of a secondary battery, there is used a charge control circuit that controls a charge current by a current-controlling transistor composed of a MOSFET (an insulating gate-type field effect transistor: hereinafter, referred to as a MOS transistor) provided between an input terminal, to which a direct-current voltage from a primary power supply such as an AC adapter is inputted, and an output terminal, to which a secondary battery is connected. Heretofore, in the charge control circuit, a current that flows through the current-controlling transistor at the time of charge has been detected, whereby control has been performed so that the charge current can be constant (refer to Japanese Patent Laid-Open Publication No. 2009-294981).
Meanwhile, as an electronic instrument equipped with a solar cell has been being widespread, there have been proposed a variety of inventions regarding a power supply device that uses a solar cell as a power supply and regarding a charge control circuit that charges a secondary battery by using a solar cell as a primary power supply (refer to Japanese Patent Laid-Open Publication No. 2010-104117).
In the solar cell, an output current and output voltage thereof are changed relatively largely in response to an amount of solar radiation and to a load. Accordingly, in the power supply device that uses the solar cell as the power supply and in the charge control device that charges the secondary battery by using the solar cell as the primary power supply, it is necessary to detect the current and the voltage. In the conventional detection of the current and the voltage, it has been general to separately provide: as shown in FIG. 8A, a voltage detection circuit including a comparator CMP1 that compares an input voltage Vs and a reference voltage Vref1 with each other; and as shown in FIG. 8B, a current detection circuit including a sensing resistor Rs, which converts an input current Is into a voltage, and a comparator CMP2 that detects a voltage drop of the resistor concerned. However, such a detection method has a problem that, since two comparators are used, a circuit scale is increased, and an increase of a chip size is brought about in the case of composing the control circuit as an IC.